Servants, be obedient - Though δουλος frequently signifies a slave or bondman, yet it often implies a servant in general, or any one bound to another, either for a limited time, or for life. Even a slave, if a Christian, was bound to serve him faithfully by whose money he was bought, howsoever illegal that traffic may be considered. In heathen countries slavery was in some sort excusable; among Christians it is an enormity and a crime for which perdition has scarcely an adequate state of punishment.
According to the flesh - Your masters in secular things; for they have no authority over your religion, nor over your souls.
With fear and trembling - Because the law gives them a power to punish you for every act of disobedience.
In singleness of your heart - Not merely through fear of punishment, but from a principle of uprightness, serving them as you would serve Christ.